When Louis Loucheur was named Minister for the Liberated Zones, Mercier accompanied him and dealt with the German factories that were dependent on the Military Control Board.

From its first holding, a 25% stake in the Turkish Petroleum Company, the CFP grew thanks to oil extraction near Kirkuk, Iraq, then in Colombia and in Venezuela.

Mercier extended the vertical integration of the company by constructing petroleum transport infrastructure and refineries at Gonfreville, near Le Havre and on the Étang de Berre, near Martigues.

In December 1925, Mercier founded the Redressement Français (literally the "French Resurgence"), a movement under the patronage of Marshal Ferdinand Foch with the goal of "gathering the elite and raising up the masses" (Kuisel 1967, p. 49).

The lack of success of his business, his involvement in the events of February 6, 1934, which he described as the victory of the "fighting spirit",[3] and the fall of the national union government of Gaston Doumergue (November 1934) all certainly drove Mercier to dissolve the Redressement Français in 1935.

He then ceased to be the spokesman[4] of the Polytechnique modernisers, and that role was passed on to Louis Marlio and to Auguste Detœuf, author of the magazine Nouveaux Cahiers.

Although the Vichy regime include many former members of the RF, such as Raphaël Alibert (justice minister) or Hubert Lagardelle, Ernest Mercier did not collaborate.

He believed that it was partly in revenge for this that Yves Bouthillier (a former RF member who had become the Vichy finance minister) had created legislation limiting the number of administrative posts that one person could occupy (Kuisel, 1967, p. 148), which forced Mercier to leave the CFP.

He continued to preside over the French branch of the International Chamber of Commerce and sat on the boards of several companies, while as an engineer he pursued research on electric turbines.